Many industrial applications, such as coal conversion plants, paper mills, cement manufacturing plants, and sewage treatment plants have difficulty in reliably measuring the flow of a liquid type slurry through a pipe. Part of the problem is that the slurry temperature and/or pressure can be quite high, the slurry can be corrosive or abrasive, or the slurry can be of high viscosity. Conventional flowmeters requiring structure located inside of the pipe or having takeoff ports in the pipe walls generally prove inappropriate as such flowmeters tend to foul up in a very short time. Flowmeters that use structure located only outside of the pipe, including the electromagnetic flowmeter, the thermal flowmeter, and the sonic flowmeter, are available and have various degrees of appeal. The electromagnetic flowmeter however, requires that the liquid conveyed be electrically conductive in order to detect flow movement. In the thermal flowmeter, heat is applied to the moving slurry between a pair of axially spaced sensors, and the temperature differential is sensed. This flowmeter proves inappropriate where the liquid temperature itself is extremely high or where the heat loss from the pipe is high. The sonic flowmeter utilizes the possible shift in the time or location of a sonic signal through the slurry as a function of the slurry velocity, and the one type includes the Doppler flowmeter.
An ultrasonic flowmeter of the Doppler type would have two transducer units bonded or mechanically held tightly against the outside walls of the pipe. A constant frequency ultrasonic signal (500,000 Hz, for example) from the one transducer unit is transmitted through the pipe wall obliquely into the flow stream and is scattered off the particles moving therein, and the scattered signal is detected by the other transducer unit. In theory, the detected signal will have a shift in frequency to higher or lower than that of the original signal, depending on whether the signal is sent out against or in the same direction respectively, as the direction of the slurry flow. The Doppler frequency shift is proportional to the operating frequency and to the ratio of the vector components of the slurry velocity in the direction of the wave propagation at a velocity. Thus EQU F.sub.d =2F.sub.o (V/C) cos .theta.
where
F.sub.d =Doppler frequency shift PA0 F.sub.o =Sending transducer frequency PA0 V=Velocity of slurry PA0 C=Velocity of wave PA0 .theta.=Angle of wave propagation relative to the axial flow.
The use of ultrasonic waves is attractive because of the relatively low propagation velocity and the ability to penetrate solid pipes and opaque slurries. However, as there are many particles moving with the slurry, the detected frequency shift signal therefore is received off of many particles and is in the form of a broadband of many frequencies. Correlating the frequency shift signal to the velocity flow thus becomes very difficult. Moreover, inasmuch as the Doppler frequency shift is a function of the velocity of the slurry flow compared to the velocity of the ultrasonic signal in the slurry, the frequency shift is quite small, of the order of 0.01-1.0% of the original ultrasonic signal.